The present invention relates to surgical navigation systems, sometimes called localization devices. More particularly, the present invention relates to methods and apparatus for positioning a bone prosthesis during orthopedic surgery using a surgical navigation system.
Many different types of surgical navigation systems are known, including surgical navigation systems that rely on optical, electromagnetic, mechanical, ultrasonic, and gyroscopic position and/or orientation sensing techniques and apparatus. Optical-, electromagnetic-, mechanical-, and ultrasonic-based surgical navigation systems are well known at this time. Gyroscopic-based surgical navigation systems are, perhaps, not as well established. Gyroscopic systems use inertial sensors. In gyroscopic systems, each marker comprises one or more gyroscopes. If six gyroscopic sensors are incorporated on a rigid body marker, namely, three sensors for detecting force and/or acceleration in linear directions and three sensors for determining rotational forces or acceleration, a marker can be tracked in all six degrees of freedom. Note that gyroscopic sensors do not provide information as to position per se, but to acceleration or movement. The positions and/or orientations of such sensors are not determined directly in a coordinate system, but are calculated from changes in position and orientation.
In an exemplary infrared, optical surgical navigation system 100 such as illustrated in FIG. 1, at least two sensors 114a, 114b (e.g., infrared cameras) mounted in a housing 128 are used to detect a plurality of markers 116a, 116b, 116c, 116d, 116e that can be mounted on the patient's bones 105a, 105b and/or on surgical tools 124. More particularly, the cameras 114a, 114b are coupled to a computer 112 that analyzes the images obtained by the cameras and detects the positions and orientations of the various bones and/or tools bearing the markers during the surgery and calculates and displays useful information for performing the surgery to the surgeon on a monitor 122. The computer system may be provided in a portable cart 108 and may include a memory 110 for storing data, a keyboard 120, and/or foot pedals 118 for entering data. Typically two or more of the markers 116a-116e are used simultaneously. One such surgical navigation system is the OrthoPilot available from Aesculap, Inc. of Center Valley, Pa., USA.
Each marker 116 comprises a base with a mounting mechanism 217 on one end for mounting to a complementary mounting mechanism 201 on a piece of medical equipment such as surgical pointer 124, a bone screw, or a cutting jig. Extending from the other end of the base are at least three infrared LED transmitters 208. Alternately, instead of transmitters, the system could utilize markers 116a bearing infrared reflectors 208a, as shown in FIG. 2B, which illustrates an exemplary marker 116a of the reflector type. When using reflectors, the surgical navigation system includes an infrared light source 107 directed towards the surgical field so that the reflectors 208 will reflect infrared light back to the two cameras 114a, 114b. With at least three transmitters 208 (or reflectors 208a) per marker and at least two cameras, sufficient information is available to the computer to determine the exact position and orientation of each marker 116 (or 116a) in all six degrees of freedom (e.g., x, y, z, coordinates and roll pitch and yaw angles).
The mounting mechanism at the end of the base of the marker is designed to mate with a complementary mounting mechanism on the surgical instrument in only one position and orientation. The computer is preprogrammed with information relating to the position of the operational portion of the medical instrument relative to the position of the marker when mounted on it. In this manner, by detecting the position and orientation of the marker, the computer will also know the position and orientation of the medical instrument and its operational portion. For instance, the medical instrument may be the pointer 124 shown in FIG. 1 having a tip 124a, the exact position of which is known relative to the marker 116a. 
In most surgical navigation procedures, it is necessary to discern the markers 116 or 116a from each other. This can be done in several different ways. If LED transmitters are used, each transmitter 208 can be timed to emit light only during a specific time interval that the computer knows is the time interval assigned to that particular transmitter on that particular marker. The LEDs are illuminated in sequence at a very high rate so that the computer has virtually continuous information as to the exact location of every LED. Alternately, when using reflectors, each marker 116a may have its three or more reflectors 208a positioned in slightly different relative positions to each other so that the computer can discern which marker it is observing by determining the geometric relationship between the three or more reflectors 208a on the marker 116a. 
The markers 116 are fixedly mounted on bones 105 (via bone screws) and or medical instruments 124 (FIG. 1) or 202 (FIG. 2A) positioned within the field of view of the cameras 114a, 114b so that the computer 112 can track the location and orientation of those bones and/or medical instruments. The computer will then generate useful information to help the surgeon determine appropriate locations or alignments for prosthetic implants, cutting jigs, and the like and display it in a display 123 on the monitor 122.
One known use for surgical navigation systems is in total hip joint replacement surgery. In total hip joint replacement surgery, for instance, the patient's hip joint is replaced with prosthetic components including a prosthetic cup (essentially a ball socket) mounted on the pelvis in the reamed acetabulum and a prosthetic femoral component, comprising a ball shaped head mounted on a stem. The stem is inserted in a prepared channel along the femoral canal. The ball will fit in the cup to form the new, prosthetic hip joint.
Presently, there are several surgical navigation systems on the market that offer a total hip replacement module. Depending on the surgical navigation system and software, the system is used to assist in the positioning of only the pelvic component, i.e., the cup, while the femoral component is positioned manually or the surgical navigation system is used to assist in the positioning of both the cup and the femoral component.
Some of the most important positioning parameters for hip joint replacement are (1) lengthening/shortening of the leg (also referred to as cranialization/caudalization), (2) medialization/lateralization of the leg and/or femoral offset, and (3) range of motion.
Lengthening/shortening refers to the change, if any, in the length of the patient's leg after the prosthetic hip joint has been installed. Usually, it is desirable for both of the patient's legs to be of equal length after the surgery. Often, this means that there should be no lengthening or shortening. However, often the legs are not of equal length before the surgery and, thus, it may be desirable to increase or decrease the patient's leg length.
There is no clear agreement in the profession as to an exact definition of either medialization/lateralization or femoral offset. However, generally they are measurements indicative of the distance that the muscles that attach to the pelvis must traverse. Femoral offset generally is a feature of the femur itself, whereas medialization/lateralization is a feature of the entire joint, including the pelvis and the femur. For purposes of this discussion, we shall consider femoral offset to be the shortest distance between the femoral mechanical axis (i.e., the line connecting the center of rotation of the hip joint to the center of rotation of the knee joint) and the point where the femoral canal and the femoral neck cross each other. The offset distance typically is in the direction of the femoral neck, which typically is at an angle of about 15 degrees to the frontal plane when viewed in a horizontal plane (i.e., a plane perpendicular to the cranial caudal direction). For purposes of this discussion, medialization/lateralization is the sum of (1) the change in position of the center of rotation of the hip joint in the medial/lateral plane (by virtue of reaming of the acetabulum and implantation of a prosthetic cup, for instance) and (2) the change in femoral offset (by virtue of the removal of the top of the femur and implantation of a prosthetic stem and ball, for instance).
If the final configuration of the joint and bones results in an overall medialization (As compared to the original configuration), the muscles that attach to the pelvis will have reduced tension and a reduced lever arm, leading to weakness in the leg. On the other hand, if the final configuration of the joint and bones results in an overall lateralization, the lever arm and tension is increased, which generally dictates greater patient leg strength as well as longer life of the prostheses.
Finally, range of motion refers to the angle to which the femur can be bent before the stem of the femoral implant impinges on the pelvis. Of course, it is desirable for the patient to have about the same range of motion after the surgery as before.
In the surgical navigation systems that assist surgeons in the placement of both the pelvic implant and the femoral implant, a marker that can be tracked by the surgical navigation system is mounted to each of the pelvis and the femur. Various anatomical landmarks on the pelvis are palpated and recorded in the pelvic frame of reference (i.e., relative to the pelvic marker). Likewise, various anatomical landmarks on the femur are recorded in the femoral frame of reference (ie., relative to the femoral marker). Furthermore, the relationship of the pelvic marker and the femoral marker are registered to one another so that the pelvic frame of reference and the femoral frame of reference can be registered to one another in the overall frame of reference of the surgical navigation system.
During cup navigation, the surgical navigation system tracks the pelvic marker as well as a second marker mounted on a surgical instrument, such as a reamer for reaming the acetabulum for accepting the prosthetic cup or to the cup implant itself, either directly or via an impacter rigidly coupled to the cup. The surgical navigation system is programmed to determine the position and orientation of the cup relative to the various anatomical landmarks on the pelvis. The orientation and position of the surgical instrument and/or implant relative to the pelvis is tracked and recorded by the surgical navigation system. The surgical navigation system will display on the monitor various data to help the surgeon affix the cup in the proper position and orientation relative to both the pelvis and the femur. Such relevant information may include the position of the center of rotation of the cup, and the angular orientation of the cup in both anteversion and inclination. This can be done, for instance, by tracking the reamer, and particularly, its final position and orientation at the end of the reaming, since the position and orientation of the reamer at the end of the reaming process largely dictates the position and orientation that the cup implant will take when implanted.
It typically will also be desirable to subsequently track the cup directly during its implantation (via a marker mounted to the cup either directly or via the impacter tool that usually is rigidly affixed to the cup during its implantation). Specifically, while the reaming operation essentially dictates the final coordinate position of the cup and cannot be changed during cup implantation, the orientation of the cup actually can be affected during implantation of the cup. Therefore, it is advisable to also track the cup during implantation and display at least its anteversion and inclination angles, if not its coordinate position.
Similarly, during navigation of the femoral implant, the surgical navigation system tracks the femoral marker and a second marker attached to a surgical instrument, such as a rasp for creating the channel within which the femoral component will be implanted and/or the stem portion of the femoral implant itself. The pelvis does not need to be tracked. However, since the surgeon has already implanted the cup in the pelvis and the system has recorded the position of the cup, the surgical navigation system also may track the pelvis (via the pelvic marker) during the femoral implantation stage of the procedure. The surgical navigation system can display to the surgeon relevant information for properly orienting and positioning the femoral implant. This information may include not only the parameters determined by the positional relationship of the femoral implant to the femur, but also the global changes of the hip joint based on the position and orientation of the cup implant and femoral implant with respect to various landmarks on the pelvis and/or femur. This information may include, for instance, anteversion and inclination angles for the joint, leg lengthening/shortening, leg medialization/lateralization, and range of motion.
In order for conventional surgical navigation systems to provide the navigational assistance with respect to the placement of the femoral implant as outlined above, a marker must be mounted on the femur. Placement of a marker on the femur is time-consuming. Additionally, the need to place a marker on the femur requires either an additional incision during the surgical procedure or additional space within the surgical approach and, therefore, a bigger skin cut in order to accommodate it. This is contradictory to the minimally invasive approach desired in modern orthopedic surgical procedures.
Accordingly, it is an object of the present invention to provide an improved method and apparatus for surgical navigation.
It is another object of the present invention to provide an improved method and apparatus for performing hip joint replacement surgery using a surgical navigation system.
It is a further object of the present invention to provide an improved method and apparatus for performing hip joint replacement surgery using a surgical navigation system that eliminates the need to affix a marker to the femur.